Thermodynamics of Intercalation of Bromine in Graphite
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S. H. ANDERSON AND D. D. L. CHUNG Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213, USA
ABSTRACT The thermodynamics of intercalation of bromine in highly oriented pyrolytic graphite had been studied by determining the pressure-temperature equilibrium diagram for stages 2-4. The standard heat and entropy of reaction for the transformation from stage n to stage n-l (n=5, 4, 3) were found to be roughly the same, though the enthalpy of reaction became slightly more negative as the stage number increased. The heat and entropy of formation from pure graphite were thus found to be -10.9 kcal mol- Br2 and -30.4 cal mol-1 Br 2 K-1 respectively for stage 2, -11.3 kcal mol-1 Br and -30.6 cal 1 mol-1 Br 2 K7 respectively for stage 3, and -il.5 kcal mol-I Br2 and -30.6 cal mol-I Br2 K71 respectively for stage 4.
INTRODUCTION The thermodynamics of intercalation of alkali metals in graphite had been studied for stages 1-5 by vapor pressure measurement [1-4], solid-state emf measurement 15], calorimetry 16) and stage-2-to-stage-i transformation temperature measurement 17). On the other hand, the thermodynamics of intercalation of bromine in graphite had only been studied by decomposition vapor pressure measurement of stage 2 [8-10], which gave the standard heat of reaction of -10.2 ± 0.3 kcal K71 for the mol-1 and the standard entropy of reaction of -29.9 - 1.2 cal molstage-3-to-stage-2 transformation [10]. In this paper, by determining the pressure-temperature equilibrium diagram for stages 2-4 of graphite-bromine, we have obtained the standard heats of reaction and the standard entropies of reactionfDr the transformations from stage 5 to stage 4, from stage 4 to stage 3, and from stage 3 to stage 2. Using these data, the heats of formation of stages 2-4 were calculated. INTERCALATION METHODS Four methods of intercalation have been investigated. They differ in the parameter(s) used to control the eventual stage, as listed in Table 1 and also described below. 1. Temperature method (two-bulb). In this method, the sample temperature is varied to control the eventual stage, while the intercalate vapor pressure resulting from a reservoir of pure intercalate is fixed. The sample temperature should be kept higher than the intercalate reservoir temperature to avoid condensation of the intercalate on the sample. This method is popular and was originally developed for the intercalation of alkali metals [7]. *Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under Grant No. AFOSR-78-3536. The United States Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation hereon.
Mat. Res. Soc. Symp. Proc. Vol.
20 (1983) @Elsevier Science Publishing Co.,
Inc.
272 TABLE I Intercalation Mlethods Parameters Intercalate vapor pressure Sample temperature
Method Temperature method (two-bulb) Temperature method (one-bulb) Solution method Temperature-solution method X Va
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